1. Field of the Invention
The present invention relates to a surface light source device of an edge light system and a light guide used therefor, in particular, to a surface light source device designed to reduce visibility of brightness unevenness and a light guide used therefor. The surface light source device of the present invention is suitable, for example, for a backlight of a liquid crystal display device used as a monitor of a portable notebook personal computer and a display part of a liquid crystal television or a video integrated liquid crystal television. Further, the device of the present invention is suitable for a backlight of a liquid crystal display device of a comparatively small size used as a display panel of portable type electronic equipment such as a portable telephone and an indicator of a variety of types of equipment. Furthermore, the device of the present invention is suitable for a backlight of a liquid crystal display device used as a guide display board or a signboard in a station or a public facility. Further, the device of the present invention is suitable for a backlight of a liquid crystal display device used as a marking device of a traffic sign or the like on a superhighway or an ordinary road.
2. Description of the Related Art
A liquid crystal display device has been widely used as a monitor of a portable notebook personal computer or the like, or a display part of a liquid crystal television and a video integrated liquid crystal television, and further in a variety of other fields. The liquid crystal display device is basically configured with a backlight and a liquid crystal display element. As the backlight, an edge light type has been frequently used in view of reducing diameter of the liquid crystal display device. The widely used backlight of edge light type has a configuration in which at least one end face of a light guide in a rectangular plate shape is used as a light incident end face, a primary light source of a linear shape or a stick shape such as a straight tube type fluorescent lamp is allocated along the light incident end face, light emitted from the primary light source is introduced to the inside of the light guide from the light incident end face of the light guide, and the light exits from a light exit face, which is one of two principal faces of the light guide.
In such a backlight, unevenness may occur in a brightness distribution of a light emitting face (a brightness uniformity may be reduced), due to a propagation form of light generated by the primary light source and emitted through the light guide. As one form of the reduction in brightness uniformity, brightness of a region in the vicinity of the primary light source becomes higher than other regions.
As a technique for preventing the reduction in brightness uniformity as described above, for example, JP-Y-40-26083 (Patent Document 1), JP-U-60-60788 (Patent Document 2), and JP-U-62-154422 (Patent Document 3) disclose allocation of a film having a light absorption property or a light beam adjusting film for restricting light transmission in the vicinity of the primary light source on a light exit face of the light guide. This technique is merely for limiting light emission from a light exit face area having a small distance from the primary light source to cope with that intensity of light emitted from the light exit face of the light guide in a region in the vicinity of the primary light source is larger than intensity of light emitted from a region far from the primary light source.
In recent years, the liquid crystal display device has been required to have a proportion of display screen dimensions with respect to outer shape dimensions of the liquid crystal device as large as possible so that display efficiency is improved. Therefore, with respect to the surface light source device as well, a proportion of light emitting face dimensions with respect to outer dimensions has required to be made as large as possible, that is, dimensions of a structural part (sometimes called “frame”) existing in a frame shape in the periphery of the light emitting face has been required to be made as small as possible.
On the other hand, the surface light source device is required to be thin. In response to such a requirement, the light guide needs to be made thinner. Due to the thinning of the light guide (for example, a thickness of about 0.5 mm to 3 mm), as a special form of the reduction in brightness uniformity, a bright part in a streak shape (emission line) which is brighter than a surrounding part is sometimes observed in parallel with the light incident end face of the light guide in accordance with a light exit face position in the vicinity (for example, about 2 mm) of the light incident end face. That is, the effect in which light emitted from the primary light source functions as a secondary light source at a light guide ridge line which makes a boundary between the light incident end face and the light exit face of the light guide appears as brightness of the light emitting face of the surface light source device. This effect significantly appears mainly in a region close to the light incident end face. This phenomenon is not a problem in actuality when a width of the frame is large. However, in case of the surface light source device having the frame with a small width as described above, there is a problem that brightness unevenness due to the effect is readily visually identified. When a technique described in the above Patent Documents 1 to 3 is adopted to prevent the reduction in brightness uniformity due to the emission line, there is a problem that brightness of not only the emission line, but also of the entire surrounding part of the emission line is readily reduced.
On the other hand, due to the generation of the emission line as described above, a dark part in a streak shape (a dark line), which is darker than a surrounding part, is sometimes observed in parallel with the light incident end face. JP-A-08-227074 (Patent Document 4) describes that, as a technique for preventing generation of such a dark line, a light absorption layer having a light absorption pattern in which a light absorption rate gradually decreases as the light absorption layer becomes far from the light incident end face is formed.
In the technique of Patent Document 4 described above, a pattern such as a dotted pattern is adopted as a light absorption pattern. However, in this case, there partially exists a region that does not absorb light, and light shielding in this region is insufficient. As a result, there is a problem that an emission line is observed in this region.
As described above, as one form in which brightness unevenness of the light emitting face is readily visually identified, a part with high brightness (emission line or emission band) and a part with low brightness (dark line or dark band) are generated in a specific interval in a region in the vicinity of the light incident end face, and such parts are visually identified as a plurality of light and dark lines extending in substantial parallel with the light incident end face.
As a technique for preventing the brightness unevenness in the vicinity of the light incident end face as described above, for example, JP-A-10-153778 (Patent Document 5) discloses that a light diffusion region in a band shape having a higher light scattering property than an adjacent region is provided in a region in the vicinity of the light incident end face of the light exit face. In addition, in order to achieve a similar object, for example, JP-A-2002-216530 (Patent Document 6) discloses that an average inclination angle of a surface of a region in the vicinity of the light incident end face of the light exit face is made larger than other regions of the light exit face.
According to the techniques in Patent Document 5 and Patent Document 6 described above, a light scattering property and a light diffusion property in a region in the vicinity of the light incident end face of the light exit face are intensified to increase an amount of exit light in this area. In this manner, a dark band is made inconspicuous so that brightness unevenness is reduced.
As another form of the reduction in brightness uniformity, brightness in the vicinity of two end faces adjacent to the light incident end face of the light guide is lower than other regions of the light exit face. This form is conspicuous especially in a region of the light exit face in the vicinity of corner parts of the light guide where the two end faces described above and the light incident end face intersect with each other.
As described above, dimensions of the frame are required to be made smaller, and dimensions of the primary light source in a linear shape or a stick shape allocated along the light incident end face of the light guide need to be reduced. The reduction of dimensions is achieved by reducing, that is, thinning a width of the primary light source on one hand, and is achieved by reducing a length of the primary light source, that is, making the length of the primary light source shorter than the light incident end face of the light guide, where the primary light source faces, on the other hand.
However, when an attempt is made to reduce the length of the primary light source, a problem to be described below occurs. That is, a socket is attached to at least one end part of the primary light source, and this end part is a non-light emitting part that does not emit light in practice. For this reason, the non-light emitting part exists at a position facing the light incident end face as a result of the reduction of dimensions, and an amount of light emitted from the light exit face at corner parts of the light guide corresponding to the non-light emitting part becomes little. Then, brightness of this part is lowered and brightness unevenness is generated.
With the aim of resolving the problems described above, for example, JP-A-2002-169033 (Patent Document 7) discloses that fine angular grains are provided in a superposed manner in regions, for example, both corners on a light incident side or a side edge part, where brightness is relatively reduced in the light exit face of the light guide. In addition, for example, JP-A-2002-258057 (Patent Document 8) discloses that an average inclination angle in the vicinity of a corner part where brightness is relatively reduced in the light exit face of the light guide is made larger than average inclination angles of other parts.
Patent Document 1: JP-Y-40-26083
Patent Document 2: JP-U-60-60788
Patent Document 3: JP-U-62-15442
Patent Document 4: JP-A-08-227074
Patent Document 5: JP-A-10-153778
Patent Document 6: JP-A-2002-216530
Patent Document 7: JP-A-2002-169033
Patent Document 8: JP-A-2002-258057
An object of the present invention is to provide a surface light source device in which brightness unevenness is not readily visually identified and a light guide used therefor. In particular, such brightness unevenness includes brightness unevenness in a region in the vicinity of the light incident end face of the light guide that occurs due to the reduction in thickness of the light guide, or brightness unevenness in a region of the light exit face in the vicinity of a corner part of the light guide in the vicinity of the light incident end face that occurs due to the reduction of dimensions of the frame.